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11 <H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
13 <P>Last updated: Sun Oct 20 20:47:14 EDT 2002</P>
15 <P>Current maintainer: Bruce Momjian (<A href=
16 "mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR>
19 <P>The most recent version of this document can be viewed at <A
21 "http://www.PostgreSQL.org/docs/faq-english.html">http://www.PostgreSQL.org/docs/faq-english.html</A>.</P>
23 <P>Platform-specific questions are answered at <A href=
24 "http://www.PostgreSQL.org/users-lounge/docs/faq.html">http://www.PostgreSQL.org/users-lounge/docs/faq.html</A>.</P>
27 <H2 align="center">General Questions</H2>
28 <A href="#1.1">1.1</A>) What is PostgreSQL? How is it pronounced?<BR>
29 <A href="#1.2">1.2</A>) What is the copyright on PostgreSQL?<BR>
30 <A href="#1.3">1.3</A>) What Unix platforms does PostgreSQL run
32 <A href="#1.4">1.4</A>) What non-Unix ports are available?<BR>
33 <A href="#1.5">1.5</A>) Where can I get PostgreSQL?<BR>
34 <A href="#1.6">1.6</A>) Where can I get support?<BR>
35 <A href="#1.7">1.7</A>) What is the latest release?<BR>
36 <A href="#1.8">1.8</A>) What documentation is available?<BR>
37 <A href="#1.9">1.9</A>) How do I find out about known bugs or
39 <A href="#1.10">1.10</A>) How can I learn <SMALL>SQL</SMALL>?<BR>
40 <A href="#1.11">1.11</A>) Is PostgreSQL Y2K compliant?<BR>
41 <A href="#1.12">1.12</A>) How do I join the development team?<BR>
42 <A href="#1.13">1.13</A>) How do I submit a bug report?<BR>
43 <A href="#1.14">1.14</A>) How does PostgreSQL compare to other
44 <SMALL>DBMS</SMALL>s?<BR>
45 <A href="#1.15">1.15</A>) How can I financially assist
49 <H2 align="center">User Client Questions</H2>
50 <A href="#2.1">2.1</A>) Are there <SMALL>ODBC</SMALL> drivers for
52 <A href="#2.2">2.2</A>) What tools are available for using
53 PostgreSQL with Web pages?<BR>
54 <A href="#2.3">2.3</A>) Does PostgreSQL have a graphical user
55 interface? A report generator? An embedded query language
57 <A href="#2.4">2.4</A>) What languages are available to
58 communicate with PostgreSQL?<BR>
61 <H2 align="center">Administrative Questions</H2>
62 <A href="#3.1">3.1</A>) How do I install PostgreSQL somewhere other
63 than <I>/usr/local/pgsql</I>?<BR>
64 <A href="#3.2">3.2</A>) When I start <I>postmaster</I>, I get a
65 <I>Bad System Call</I> or core dumped message. Why?<BR>
66 <A href="#3.3">3.3</A>) When I try to start <I>postmaster</I>, I
67 get <I>IpcMemoryCreate</I> errors. Why?<BR>
68 <A href="#3.4">3.4</A>) When I try to start <I>postmaster</I>, I
69 get <I>IpcSemaphoreCreate</I> errors. Why?<BR>
70 <A href="#3.5">3.5</A>) How do I control connections from other
72 <A href="#3.6">3.6</A>) How do I tune the database engine for
73 better performance?<BR>
74 <A href="#3.7">3.7</A>) What debugging features are available?<BR>
75 <A href="#3.8">3.8</A>) Why do I get <I>"Sorry, too many
76 clients"</I> when trying to connect?<BR>
77 <A href="#3.9">3.9</A>) What is in the <I>pgsql_tmp</I>
79 <A href="#3.10">3.10</A>) Why do I need to do a dump and restore
80 to upgrade PostgreSQL releases?<BR>
83 <H2 align="center">Operational Questions</H2>
84 <A href="#4.1">4.1</A>) What is the difference between binary
85 cursors and normal cursors?<BR>
86 <A href="#4.2">4.2</A>) How do I <SMALL>SELECT</SMALL> only the
87 first few rows of a query?<BR>
88 <A href="#4.3">4.3</A>) How do I get a list of tables or other
89 things I can see in <I>psql</I>?<BR>
90 <A href="#4.4">4.4</A>) How do you remove a column from a
92 <A href="#4.5">4.5</A>) What is the maximum size for a row, a
93 table, and a database?<BR>
94 <A href="#4.6">4.6</A>) How much database disk space is required
95 to store data from a typical text file?<BR>
96 <A href="#4.7">4.7</A>) How do I find out what tables, indexes,
97 databases, and users are defined?<BR>
98 <A href="#4.8">4.8</A>) My queries are slow or don't make use of
100 <A href="#4.9">4.9</A>) How do I see how the query optimizer is
101 evaluating my query?<BR>
102 <A href="#4.10">4.10</A>) What is an R-tree index?<BR>
103 <A href="#4.11">4.11</A>) What is the Genetic Query Optimizer?<BR>
104 <A href="#4.12">4.12</A>) How do I perform regular expression
105 searches and case-insensitive regular expression searches? How do I
106 use an index for case-insensitive searches?<BR>
107 <A href="#4.13">4.13</A>) In a query, how do I detect if a field
108 is <SMALL>NULL</SMALL>?<BR>
109 <A href="#4.14">4.14</A>) What is the difference between the
110 various character types?<BR>
111 <A href="#4.15.1">4.15.1</A>) How do I create a
112 serial/auto-incrementing field?<BR>
113 <A href="#4.15.2">4.15.2</A>) How do I get the value of a
114 <SMALL>SERIAL</SMALL> insert?<BR>
115 <A href="#4.15.3">4.15.3</A>) Don't <I>currval()</I> and
116 <I>nextval()</I> lead to a race condition with other users?<BR>
117 <A href="#4.15.4">4.15.4</A>) Why aren't my sequence numbers
118 reused on transaction abort? Why are there gaps in the numbering of
119 my sequence/SERIAL column?<BR>
120 <A href="#4.16">4.16</A>) What is an <SMALL>OID</SMALL>? What is a
121 <SMALL>TID</SMALL>?<BR>
122 <A href="#4.17">4.17</A>) What is the meaning of some of the terms
123 used in PostgreSQL?<BR>
124 <A href="#4.18">4.18</A>) Why do I get the error <I>"ERROR: Memory
125 exhausted in AllocSetAlloc()"</I>?<BR>
126 <A href="#4.19">4.19</A>) How do I tell what PostgreSQL version I
128 <A href="#4.20">4.20</A>) Why does my large-object operations get
129 <I>"invalid large obj descriptor"</I>?<BR>
130 <A href="#4.21">4.21</A>) How do I create a column that will
131 default to the current time?<BR>
132 <A href="#4.22">4.22</A>) Why are my subqueries using
133 <CODE><SMALL>IN</SMALL></CODE> so slow?<BR>
134 <A href="#4.23">4.23</A>) How do I perform an outer join?<BR>
135 <A href="#4.24">4.24</A>) How do I perform queries using multiple
137 <A href="#4.25">4.25</A>) How do I return multiple rows or columns
139 <A href="#4.26">4.26</A>) Why can't I reliably create/drop
140 temporary tables in PL/PgSQL functions?<BR>
141 <A href="#4.27">4.27</A>) What replication options are available?<BR>
142 <A href="#4.28">4.28</A>) What encryption options are available?<BR>
145 <H2 align="center">Extending PostgreSQL</H2>
146 <A href="#5.1">5.1</A>) I wrote a user-defined function. When I run
147 it in <I>psql</I>, why does it dump core?<BR>
148 <A href="#5.2">5.2</A>) How can I contribute some nifty new types
149 and functions to PostgreSQL?<BR>
150 <A href="#5.3">5.3</A>) How do I write a C function to return a
152 <A href="#5.4">5.4</A>) I have changed a source file. Why does the
153 recompile not see the change?<BR>
157 <H2 align="center">General Questions</H2>
159 <H4><A name="1.1">1.1</A>) What is PostgreSQL? How is it pronounced?</H4>
161 <P>PostgreSQL is pronounced <I>Post-Gres-Q-L</I>.</P>
163 <P>PostgreSQL is an enhancement of the POSTGRES database management
164 system, a next-generation <SMALL>DBMS</SMALL> research prototype.
165 While PostgreSQL retains the powerful data model and rich data
166 types of POSTGRES, it replaces the PostQuel query language with an
167 extended subset of <SMALL>SQL</SMALL>. PostgreSQL is free and the
168 complete source is available.</P>
170 <P>PostgreSQL development is performed by a team of
171 developers who all subscribe to the PostgreSQL development mailing
172 list. The current coordinator is Marc G. Fournier (<A href=
173 "mailto:scrappy@PostgreSQL.org">scrappy@PostgreSQL.org</A>). (See
174 section <a href="#1.6">1.6</a> on how to join). This team is now
175 responsible for all development of PostgreSQL.</P>
177 <P>The authors of PostgreSQL 1.01 were Andrew Yu and Jolly Chen.
178 Many others have contributed to the porting, testing, debugging,
179 and enhancement of the code. The original Postgres code, from which
180 PostgreSQL is derived, was the effort of many graduate students,
181 undergraduate students, and staff programmers working under the
182 direction of Professor Michael Stonebraker at the University of
183 California, Berkeley.</P>
185 <P>The original name of the software at Berkeley was Postgres. When
186 <SMALL>SQL</SMALL> functionality was added in 1995, its name was
187 changed to Postgres95. The name was changed at the end of 1996 to
190 <H4><A name="1.2">1.2</A>) What is the copyright on
193 <P>PostgreSQL is subject to the following COPYRIGHT:</P>
195 <P>PostgreSQL Data Base Management System</P>
197 <P>Portions copyright (c) 1996-2002, PostgreSQL Global Development
198 Group Portions Copyright (c) 1994-6 Regents of the University of
201 <P>Permission to use, copy, modify, and distribute this software
202 and its documentation for any purpose, without fee, and without a
203 written agreement is hereby granted, provided that the above
204 copyright notice and this paragraph and the following two
205 paragraphs appear in all copies.</P>
207 <P>IN NO EVENT SHALL THE UNIVERSITY OF CALIFORNIA BE LIABLE TO ANY
208 PARTY FOR DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL
209 DAMAGES, INCLUDING LOST PROFITS, ARISING OUT OF THE USE OF THIS
210 SOFTWARE AND ITS DOCUMENTATION, EVEN IF THE UNIVERSITY OF
211 CALIFORNIA HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.</P>
213 <P>THE UNIVERSITY OF CALIFORNIA SPECIFICALLY DISCLAIMS ANY
214 WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
215 OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE
216 SOFTWARE PROVIDED HEREUNDER IS ON AN "AS IS" BASIS, AND THE
217 UNIVERSITY OF CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE,
218 SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.</P>
220 <P>The above is the BSD license, the classic open-source license.
221 It has no restrictions on how the source code may be used. We like
222 it and have no intention of changing it.</P>
224 <H4><A name="1.3">1.3</A>) What Unix platforms does PostgreSQL run
227 <P>In general, a modern Unix-compatible platform should be able to
228 run PostgreSQL. The platforms that had received explicit testing at
229 the time of release are listed in the installation
232 <H4><A name="1.4">1.4</A>) What non-Unix ports are available?</H4>
234 <P><STRONG>Client</STRONG></P>
236 <P>It is possible to compile the <I>libpq</I> C library, psql, and
237 other interfaces and client applications to run on MS Windows platforms.
238 In this case, the client is running on MS Windows, and communicates
239 via TCP/IP to a server running on one of our supported Unix
240 platforms. A file <I>win32.mak</I> is included in the distribution
241 for making a Win32 <I>libpq</I> library and <I>psql</I>. PostgreSQL
242 also communicates with <SMALL>ODBC</SMALL> clients.</P>
244 <P><STRONG>Server</STRONG></P>
246 <P>The database server can run on Windows NT and Win2k using
247 Cygwin, the Cygnus Unix/NT porting library. See
248 <I>pgsql/doc/FAQ_MSWIN</I> in the distribution or the MS Windows FAQ
249 at <A href="http://www.PostgreSQL.org/docs/faq-mswin.html">
250 http://www.PostgreSQL.org/docs/faq-mswin.html</A>.</P>
252 <p>A native port to MS Win NT/2000/XP is currently being worked
255 <H4><A name="1.5">1.5</A>) Where can I get PostgreSQL?</H4>
257 <P>The primary anonymous ftp site for PostgreSQL is <A href=
258 "ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A>.
259 For mirror sites, see our main web site.</P>
261 <H4><A name="1.6">1.6</A>) Where can I get support?</H4>
263 <P>The main mailing list is: <A href=
264 "mailto:pgsql-general@PostgreSQL.org">pgsql-general@PostgreSQL.org</A>.
265 It is available for discussion of matters pertaining to PostgreSQL.
266 To subscribe, send mail with the following lines in the body (not
267 the subject line):</P>
274 "mailto:pgsql-general-request@PostgreSQL.org">pgsql-general-request@PostgreSQL.org</A>.</P>
276 <P>There is also a digest list available. To subscribe to this
277 list, send email to: <A href=
278 "mailto:pgsql-general-digest-request@PostgreSQL.org">pgsql-general-digest-request@PostgreSQL.org</A>
285 Digests are sent out to members of this list whenever the main list
286 has received around 30k of messages.
288 <P>The bugs mailing list is available. To subscribe to this list,
289 send email to <A href=
290 "mailto:pgsql-bugs-request@PostgreSQL.org">pgsql-bugs-request@PostgreSQL.org</A>
297 There is also a developers discussion mailing list available. To
298 subscribe to this list, send email to <A href=
299 "mailto:pgsql-hackers-request@PostgreSQL.org">pgsql-hackers-request@PostgreSQL.org</A>
306 <P>Additional mailing lists and information about PostgreSQL can be
307 found via the PostgreSQL WWW home page at:</P>
310 <A href="http://www.PostgreSQL.org">http://www.PostgreSQL.org</A>
313 <P>There is also an IRC channel on EFNet, channel
314 <I>#PostgreSQL</I>. I use the Unix command <CODE>irc -c
315 '#PostgreSQL' "$USER" irc.phoenix.net.</CODE></P>
317 <P>A list of commercial support companies is available at <A href=
318 "http://www.PostgreSQL.org/users-lounge/commercial-support.html">http://www.PostgreSQL.org/users-lounge/commercial-support.html</A>.</P>
320 <H4><A name="1.7">1.7</A>) What is the latest release?</H4>
322 <P>The latest release of PostgreSQL is version 7.2.3.</P>
324 <P>We plan to have major releases every four months.</P>
326 <H4><A name="1.8">1.8</A>) What documentation is available?</H4>
328 <P>Several manuals, manual pages, and some small test examples are
329 included in the distribution. See the <I>/doc</I> directory. You
330 can also browse the manual online at <A href=
331 "http://www.PostgreSQL.org/users-lounge/docs/">http://www.PostgreSQL.org/users-lounge/docs/</A>.</P>
333 <P>There are two PostgreSQL books available online at <A href=
334 "http://www.PostgreSQL.org/docs/awbook.html">http://www.PostgreSQL.org/docs/awbook.html</A>
336 "http://www.commandprompt.com/ppbook/">http://www.commandprompt.com/ppbook/</A>.
337 There is a list of PostgreSQL books available for purchase at <A
339 "http://www.ca.PostgreSQL.org/books/">http://www.ca.PostgreSQL.org/books/</A>.
340 There is also a collection of PostgreSQL technical articles at <A
342 "http://techdocs.PostgreSQL.org/">http://techdocs.PostgreSQL.org/</A>.</P>
344 <P><I>psql</I> has some nice \d commands to show information about
345 types, operators, functions, aggregates, etc.</P>
347 <P>Our web site contains even more documentation.</P>
349 <H4><A name="1.9">1.9</A>) How do I find out about known bugs or
350 missing features?</H4>
352 <P>PostgreSQL supports an extended subset of <SMALL>SQL</SMALL>-92.
353 See our <A href="http://developer.PostgreSQL.org/todo.php">TODO</A>
354 list for known bugs, missing features, and future plans.</P>
356 <H4><A name="1.10">1.10</A>) How can I learn
357 <SMALL>SQL</SMALL>?</H4>
359 <P>The PostgreSQL book at <A href=
360 "http://www.PostgreSQL.org/docs/awbook.html">http://www.PostgreSQL.org/docs/awbook.html</A>
361 teaches <SMALL>SQL</SMALL>. There is another PostgreSQL book at <A
363 "http://www.commandprompt.com/ppbook/">http://www.commandprompt.com/ppbook.</A>
364 There is a nice tutorial at <A href=
365 "http://www.intermedia.net/support/sql/sqltut.shtm">http://www.intermedia.net/support/sql/sqltut.shtm,</A>
367 "http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM">
368 http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM,</A>
370 "http://sqlcourse.com/">http://sqlcourse.com.</A></P>
372 <P>Another one is "Teach Yourself SQL in 21 Days, Second Edition"
374 "http://members.tripod.com/er4ebus/sql/index.htm">http://members.tripod.com/er4ebus/sql/index.htm</A></P>
376 <P>Many of our users like <I>The Practical SQL Handbook</I>,
377 Bowman, Judith S., et al., Addison-Wesley. Others like <I>The
378 Complete Reference SQL</I>, Groff et al., McGraw-Hill.</P>
380 <H4><A name="1.11">1.11</A>) Is PostgreSQL Y2K compliant?</H4>
382 <P>Yes, we easily handle dates past the year 2000 AD, and before
385 <H4><A name="1.12">1.12</A>) How do I join the development
388 <P>First, download the latest source and read the PostgreSQL
389 Developers documentation on our web site, or in the distribution.
390 Second, subscribe to the <I>pgsql-hackers</I> and
391 <I>pgsql-patches</I> mailing lists. Third, submit high quality
392 patches to pgsql-patches.</P>
394 <P>There are about a dozen people who have commit privileges to the
395 PostgreSQL <SMALL>CVS</SMALL> archive. They each have submitted so
396 many high-quality patches that it was impossible for the existing
397 committers to keep up, and we had confidence that patches they
398 committed were of high quality.</P>
400 <H4><A name="1.13">1.13</A>) How do I submit a bug report?</H4>
402 <P>Please visit the PostgreSQL BugTool page at <A href=
403 "http://www.PostgreSQL.org/bugs/bugs.php">http://www.PostgreSQL.org/bugs/bugs.php</A>,
404 which gives guidelines and directions on how to submit a
407 <P>Also check out our ftp site <A href=
408 "ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A> to
409 see if there is a more recent PostgreSQL version or patches.</P>
411 <H4><A name="1.14">1.14</A>) How does PostgreSQL compare to other
412 <SMALL>DBMS</SMALL>s?</H4>
414 <P>There are several ways of measuring software: features,
415 performance, reliability, support, and price.</P>
418 <DT><B>Features</B></DT>
420 <DD>PostgreSQL has most features present in large commercial
421 <SMALL>DBMS</SMALL>s, like transactions, subselects, triggers,
422 views, foreign key referential integrity, and sophisticated
423 locking. We have some features they do not have, like
424 user-defined types, inheritance, rules, and multi-version
425 concurrency control to reduce lock contention.<BR>
429 <DT><B>Performance</B></DT>
431 <DD>PostgreSQL has performance similar to other commercial and
432 open source databases. it is faster for some things, slower for
433 others. In comparison to MySQL or leaner database systems, we are
434 slower on inserts/updates because of transaction overhead. Of
435 course, MySQL does not have any of the features mentioned in the
436 <I>Features</I> section above. We are built for reliability and
437 features, though we continue to improve performance in every
438 release. There is an interesting Web page comparing PostgreSQL to
439 MySQL at <A href="http://openacs.org/philosophy/why-not-mysql.html">
440 http://openacs.org/philosophy/why-not-mysql.html</A><BR>
445 <DT><B>Reliability</B></DT>
447 <DD>We realize that a <SMALL>DBMS</SMALL> must be reliable, or it
448 is worthless. We strive to release well-tested, stable code that
449 has a minimum of bugs. Each release has at least one month of
450 beta testing, and our release history shows that we can provide
451 stable, solid releases that are ready for production use. We
452 believe we compare favorably to other database software in this
457 <DT><B>Support</B></DT>
459 <DD>Our mailing lists provide contact with a large group of developers
460 and users to help resolve any problems encountered. While we cannot
461 guarantee a fix, commercial <SMALL>DBMS</SMALL>s do not always
462 supply a fix either. Direct access to developers, the user
463 community, manuals, and the source code often make PostgreSQL
464 support superior to other <SMALL>DBMS</SMALL>s. There is
465 commercial per-incident support available for those who need it.
466 (See <A href="#1.6">FAQ section 1.6</A>.)<BR>
470 <DT><B>Price</B></DT>
472 <DD>We are free for all use, both commercial and non-commercial.
473 You can add our code to your product with no limitations, except
474 those outlined in our BSD-style license stated above.<BR>
479 <H4><A name="1.15">1.15</A>) How can I financially assist
482 <P>PostgreSQL has had a first-class infrastructure since we started
483 in 1996. This is all thanks to Marc Fournier, who has created
484 and managed this infrastructure over the years.</P>
486 <P>Quality infrastructure is very important to an open-source
487 project. It prevents disruptions that can greatly delay forward
488 movement of the project.</P>
490 <P>Of course, this infrastructure is not cheap. There are a variety
491 of monthly and one-time expenses that are required to keep it
492 going. If you or your company has money it can donate to help fund
493 this effort, please go to <A href="http://store.pgsql.com/shopping/">http://store.pgsql.com/shopping/</A>
494 and make a donation.</P>
496 <P>Although the web page mentions PostgreSQL, Inc, the
497 "contributions" item is solely to support the PostgreSQL project
498 and does not fund any specific company. If you prefer, you can also
499 send a check to the contact address.</P>
502 <H2 align="center">User Client Questions</H2>
504 <H4><A name="2.1">2.1</A>) Are there <SMALL>ODBC</SMALL> drivers
507 <P>There are two <SMALL>ODBC</SMALL> drivers available, PsqlODBC
508 and OpenLink <SMALL>ODBC</SMALL>.</P>
510 <P>PsqlODBC is included in the distribution. More information about
511 it can be gotten from <A href=
512 "ftp://ftp.PostgreSQL.org/pub/odbc/">ftp://ftp.PostgreSQL.org/pub/odbc/</A>.</P>
514 <P>OpenLink <SMALL>ODBC</SMALL> can be gotten from <A href=
515 "http://www.openlinksw.com/">http://www.openlinksw.com</A>. It
516 works with their standard <SMALL>ODBC</SMALL> client software so
517 you'll have PostgreSQL <SMALL>ODBC</SMALL> available on every
518 client platform they support (Win, Mac, Unix, VMS).</P>
520 <P>They will probably be selling this product to people who need
521 commercial-quality support, but a freeware version will always be
522 available. Please send questions to <A href=
523 "mailto:postgres95@openlink.co.uk">postgres95@openlink.co.uk</A>.</P>
525 <P>See also the <A href=
526 "http://www.postgresql.org/devel-corner/docs/programmer/odbc.html">ODBC
527 chapter of the Programmer's Guide</A>.</P>
529 <H4><A name="2.2">2.2</A>) What tools are available for using
530 PostgreSQL with Web pages?</H4>
532 <P>A nice introduction to Database-backed Web pages can be seen at:
533 <A href="http://www.webreview.com">http://www.webreview.com</A></P>
535 <P>For Web integration, PHP is an excellent interface. It is at <A
536 href="http://www.php.net">http://www.php.net</A>.</P>
538 <P>For complex cases, many use the Perl interface and CGI.pm or mod_perl.</P>
540 <H4><A name="2.3">2.3</A>) Does PostgreSQL have a graphical user
541 interface? A report generator? An embedded query language
544 <P>We have a nice graphical user interface called PgAccess which can
545 also be used as a report generator. The Web page is
546 <A href="http://www.pgaccess.org/">http://www.pgaccess.org/</A>.</P>
548 <P>We also include <I>ecpg</I>, which is an embedded SQL query
549 language interface for C.</P>
551 <H4><A name="2.4">2.4</A>) What languages are able to communicate with PostgreSQL?</H4>
558 <LI>C++ (libpq++)</LI>
560 <LI>Embedded C (ecpg)</LI>
564 <LI>Perl (DBD::Pg and perl5)</LI>
568 <LI>Python (PyGreSQL)</LI>
570 <LI>TCL (libpgtcl)</LI>
572 <LI>C Easy API (libpgeasy)</LI>
574 <LI>PHP ('pg_' functions, Pear::DB)</LI>
576 <P>Additional interfaces are available at
577 <a href="http://www.PostgreSQL.org/interfaces.html">http://www.PostgreSQL.org/interfaces.html</A>
579 <a href="http://gborg.PostgreSQL.org">http://gborg.PostgreSQL.org</A>.
583 <H2 align="center">Administrative Questions</H2>
585 <H4><A name="3.1">3.1</A>) How do I install PostgreSQL somewhere
586 other than <I>/usr/local/pgsql</I>?</H4>
588 <P>Specify the <I>--prefix</I> option when running
589 <I>configure</I>.</P>
591 <H4><A name="3.2">3.2</A>) When I start <I>postmaster</I>, I get a
592 <I>Bad System Call</I> or core dumped message. Why?</H4>
594 <P>It could be a variety of problems, but first check to see that
595 you have System V extensions installed in your kernel. PostgreSQL
596 requires kernel support for shared memory and semaphores.</P>
598 <H4><A name="3.3">3.3</A>) When I try to start <I>postmaster</I>, I
599 get <I>IpcMemoryCreate</I> errors. Why?</H4>
601 <P>You either do not have shared memory configured properly in your
602 kernel or you need to enlarge the shared memory available in the
603 kernel. The exact amount you need depends on your architecture and
604 how many buffers and backend processes you configure for
605 <I>postmaster</I>. For most systems, with default numbers of
606 buffers and processes, you need a minimum of ~1 MB. See the <A
608 "http://www.postgresql.org/idocs/index.php?kernel-resources.html">PostgreSQL
609 Administrator's Guide</A> for more detailed information about
610 shared memory and semaphores.</P>
612 <H4><A name="3.4">3.4</A>) When I try to start <I>postmaster</I>, I
613 get <I>IpcSemaphoreCreate</I> errors. Why?</H4>
615 <P>If the error message is <I>IpcSemaphoreCreate: semget failed (No
616 space left on device)</I> then your kernel is not configured with
617 enough semaphores. Postgres needs one semaphore per potential
618 backend process. A temporary solution is to start <I>postmaster</I>
619 with a smaller limit on the number of backend processes. Use
620 <I>-N</I> with a parameter less than the default of 32. A more
621 permanent solution is to increase your kernel's
622 <SMALL>SEMMNS</SMALL> and <SMALL>SEMMNI</SMALL> parameters.</P>
624 <P>Inoperative semaphores can also cause crashes during heavy
627 <P>If the error message is something else, you might not have
628 semaphore support configured in your kernel at all. See the
629 PostgreSQL Administrator's Guide for more detailed information
630 about shared memory and semaphores.</P>
632 <H4><A name="3.5">3.5</A>) How do I control connections from other
635 <P>By default, PostgreSQL only allows connections from the local
636 machine using Unix domain sockets. Other machines will not be able
637 to connect unless you add the <I>-i</I> flag to <I>postmaster</I>,
638 <B>and</B> enable host-based authentication by modifying the file
639 <I>$PGDATA/pg_hba.conf</I> accordingly. This will allow TCP/IP
642 <H4><A name="3.6">3.6</A>) How do I tune the database engine for
643 better performance?</H4>
645 <P>Certainly, indexes can speed up queries. The
646 <SMALL>EXPLAIN</SMALL> command allows you to see how PostgreSQL is
647 interpreting your query, and which indexes are being used.</P>
649 <P>If you are doing many <SMALL>INSERTs</SMALL>, consider doing
650 them in a large batch using the <SMALL>COPY</SMALL> command. This
651 is much faster than individual <SMALL>INSERTS</SMALL>. Second,
652 statements not in a <SMALL>BEGIN WORK/COMMIT</SMALL> transaction
653 block are considered to be in their own transaction. Consider
654 performing several statements in a single transaction block. This
655 reduces the transaction overhead. Also, consider dropping and
656 recreating indexes when making large data changes.</P>
658 <P>There are several tuning options. You can disable <I>fsync()</I>
659 by starting <I>postmaster</I> with a <I>-o -F</I> option. This will
660 prevent <I>fsync()</I>s from flushing to disk after every
663 <P>You can also use the <I>postmaster</I> <I>-B</I> option to
664 increase the number of shared memory buffers used by the backend
665 processes. If you make this parameter too high, the
666 <I>postmaster</I> may not start because you have exceeded your
667 kernel's limit on shared memory space. Each buffer is 8K and the
668 default is 64 buffers.</P>
670 <P>You can also use the backend <I>-S</I> option to increase the
671 maximum amount of memory used by the backend process for temporary
672 sorts. The <I>-S</I> value is measured in kilobytes, and the
673 default is 512 (i.e. 512K).</P>
675 <P>You can also use the <SMALL>CLUSTER</SMALL> command to group
676 data in tables to match an index. See the <SMALL>CLUSTER</SMALL>
677 manual page for more details.</P>
679 <H4><A name="3.7">3.7</A>) What debugging features are
682 <P>PostgreSQL has several features that report status information
683 that can be valuable for debugging purposes.</P>
685 <P>First, by running <I>configure</I> with the --enable-cassert
686 option, many <I>assert()</I>s monitor the progress of the backend
687 and halt the program when something unexpected occurs.</P>
689 <P>Both <I>postmaster</I> and <I>postgres</I> have several debug
690 options available. First, whenever you start <I>postmaster</I>,
691 make sure you send the standard output and error to a log file,
695 ./bin/postmaster >server.log 2>&1 &
698 <P>This will put a server.log file in the top-level PostgreSQL
699 directory. This file contains useful information about problems or
700 errors encountered by the server. <I>Postmaster</I> has a <I>-d</I>
701 option that allows even more detailed information to be reported.
702 The <I>-d</I> option takes a number that specifies the debug level.
703 Be warned that high debug level values generate large log
706 <P>If <I>postmaster</I> is not running, you can actually run the
707 <I>postgres</I> backend from the command line, and type your
708 <SMALL>SQL</SMALL> statement directly. This is recommended
709 <B>only</B> for debugging purposes. Note that a newline terminates
710 the query, not a semicolon. If you have compiled with debugging
711 symbols, you can use a debugger to see what is happening. Because
712 the backend was not started from <I>postmaster</I>, it is not
713 running in an identical environment and locking/backend interaction
714 problems may not be duplicated.</P>
716 <P>If <I>postmaster</I> is running, start <I>psql</I> in one
717 window, then find the <SMALL>PID</SMALL> of the <I>postgres</I>
718 process used by <I>psql</I>. Use a debugger to attach to the
719 <I>postgres</I> <SMALL>PID</SMALL>. You can set breakpoints in the
720 debugger and issue queries from <I>psql</I>. If you are debugging
721 <I>postgres</I> startup, you can set PGOPTIONS="-W n", then start
722 <I>psql</I>. This will cause startup to delay for <I>n</I> seconds
723 so you can attach to the process with the debugger, set any
724 breakpoints, and continue through the startup sequence.</P>
726 <P>The <I>postgres</I> program has <I>-s, -A</I>, and <I>-t</I>
727 options that can be very useful for debugging and performance
730 <P>You can also compile with profiling to see what functions are
731 taking execution time. The backend profile files will be deposited
732 in the <I>pgsql/data/base/dbname</I> directory. The client profile
733 file will be put in the client's current directory. Linux requires
734 a compile with <I>-DLINUX_PROFILE</I> for proper profiling.</P>
736 <H4><A name="3.8">3.8</A>) Why do I get <I>"Sorry, too many
737 clients"</I> when trying to connect?</H4>
739 <P>You need to increase <I>postmaster</I>'s limit on how many
740 concurrent backend processes it can start.</P>
742 <P>The default limit is 32 processes. You can increase it by
743 restarting <I>postmaster</I> with a suitable <I>-N</I> value or
744 modifying <I>postgresql.conf</I>.</P>
746 <P>Note that if you make <I>-N</I> larger than 32, you must also
747 increase <I>-B</I> beyond its default of 64; <I>-B</I> must be at
748 least twice <I>-N</I>, and probably should be more than that for
749 best performance. For large numbers of backend processes, you are
750 also likely to find that you need to increase various Unix kernel
751 configuration parameters. Things to check include the maximum size
752 of shared memory blocks, <SMALL>SHMMAX;</SMALL> the maximum number
753 of semaphores, <SMALL>SEMMNS</SMALL> and <SMALL>SEMMNI;</SMALL> the
754 maximum number of processes, <SMALL>NPROC;</SMALL> the maximum
755 number of processes per user, <SMALL>MAXUPRC;</SMALL> and the
756 maximum number of open files, <SMALL>NFILE</SMALL> and
757 <SMALL>NINODE</SMALL>. The reason that PostgreSQL has a limit on
758 the number of allowed backend processes is so your system won't run
759 out of resources.</P>
761 <P>In PostgreSQL versions prior to 6.5, the maximum number of
762 backends was 64, and changing it required a rebuild after altering
763 the MaxBackendId constant in
764 <I>include/storage/sinvaladt.h</I>.</P>
766 <H4><A name="3.9">3.9</A>) What is in the <I>pgsql_tmp</I> directory?</H4>
768 <P>This directory contains temporary files generated by the query
769 executor. For example, if a sort needs to be done to satisfy an
770 <SMALL>ORDER BY</SMALL> and the sort requires more space than the
771 backend's <I>-S</I> parameter allows, then temporary files are created
772 here to hold the extra data.</P>
774 <P>The temporary files are usually deleted automatically, but might
775 remain if a backend crashes during a sort. A stop and restart of the
776 <I>postmaster</I> will remove files from those directories.</P>
778 <H4><A name="3.10">3.10</A>) Why do I need to do a dump and restore
779 to upgrade between major PostgreSQL releases?</H4>
781 <P>The PostgreSQL team makes only small changes between minor releases,
782 so upgrading from 7.2 to 7.2.1 does not require a dump and restore.
783 However, major releases (e.g. from 7.2 to 7.3) often change the internal
784 format of system tables and data files. These changes are often complex,
785 so we don't maintain backward compatability for data files. A dump outputs
786 data in a generic format that can then be loaded in using the new internal
789 <P>In releases where the on-disk format does not change, the
790 <I>pg_upgrade</I> script can be used to upgrade without a dump/restore.
791 The release notes mention whether <I>pg_upgrade</I> is available for the
796 <H2 align="center">Operational Questions</H2>
798 <H4><A name="4.1">4.1</A>) What is the difference between binary
799 cursors and normal cursors?</H4>
801 <P>See the <SMALL>DECLARE</SMALL> manual page for a
804 <H4><A name="4.2">4.2</A>) How do I <SMALL>SELECT</SMALL> only the
805 first few rows of a query?</H4>
807 <P>See the <SMALL>FETCH</SMALL> manual page, or use
808 <SMALL>SELECT</SMALL> ... <SMALL>LIMIT</SMALL>....</P>
810 <P>The entire query may have to be evaluated, even if you only want
811 the first few rows. Consider using a query that has an <SMALL>ORDER
812 BY</SMALL>. If there is an index that matches the <SMALL>ORDER
813 BY</SMALL>, PostgreSQL may be able to evaluate only the first few
814 records requested, or the entire query may have to be evaluated
815 until the desired rows have been generated.</P>
817 <H4><A name="4.3">4.3</A>) How do I get a list of tables or other
818 things I can see in <I>psql</I>?</H4>
820 <P>You can read the source code for <I>psql</I> in file
821 <I>pgsql/src/bin/psql/describe.c</I>. It contains
822 <SMALL>SQL</SMALL> commands that generate the output for psql's
823 backslash commands. You can also start <I>psql</I> with the
824 <I>-E</I> option so it will print out the queries it uses to
825 execute the commands you give.</P>
827 <H4><A name="4.4">4.4</A>) How do you remove a column from a
830 <P>This functionality was added in release 7.3 with
831 <SMALL>ALTER TABLE DROP COLUMN</SMALL>. In earlier versions,
835 LOCK TABLE old_table;
836 SELECT ... -- select all columns but the one you want to remove
839 DROP TABLE old_table;
840 ALTER TABLE new_table RENAME TO old_table;
844 <H4><A name="4.5">4.5</A>) What is the maximum size for a row, a
845 table, and a database?</H4>
847 <P>These are the limits:</P>
849 Maximum size for a database? unlimited (1 TB databases exist)
850 Maximum size for a table? 16 TB
851 Maximum size for a row? 1.6TB
852 Maximum size for a field? 1 GB
853 Maximum number of rows in a table? unlimited
854 Maximum number of columns in a table? 250-1600 depending on column types
855 Maximum number of indexes on a table? unlimited
858 Of course, these are not actually unlimited, but limited to
859 available disk space and memory/swap space. Performance may suffer
860 when these values get unusually large.
862 <P>The maximum table size of 16 TB does not require large file
863 support from the operating system. Large tables are stored as
864 multiple 1 GB files so file system size limits are not
867 <P>The maximum table size and maximum number of columns can be
868 increased if the default block size is increased to 32k.</P>
870 <H4><A name="4.6">4.6</A>) How much database disk space is required
871 to store data from a typical text file?</H4>
873 <P>A PostgreSQL database may require up to five times the disk
874 space to store data from a text file.</P>
876 <P>As an example, consider a file of 100,000 lines with an integer
877 and text description on each line. Suppose the text string
878 avergages twenty bytes in length. The flat file would be 2.8 MB.
879 The size of the PostgreSQL database file containing this data can
880 be estimated as 6.4 MB:</P>
882 36 bytes: each row header (approximate)
883 24 bytes: one int field and one text field
884 + 4 bytes: pointer on page to tuple
885 ----------------------------------------
888 The data page size in PostgreSQL is 8192 bytes (8 KB), so:
891 ------------------- = 128 rows per database page (rounded down)
895 -------------------- = 782 database pages (rounded up)
898 782 database pages * 8192 bytes per page = 6,406,144 bytes (6.4 MB)
901 <P>Indexes do not require as much overhead, but do contain the data
902 that is being indexed, so they can be large also.</P>
904 <P><SMALL>NULL</SMALL>s are stored in bitmaps, so they
905 use very little space.</P>
907 <H4><A name="4.7">4.7</A>) How do I find out what tables, indexes,
908 databases, and users are defined?</H4>
910 <P><I>psql</I> has a variety of backslash commands to show such
911 information. Use \? to see them. There are also system tables
912 beginning with <I>pg_</I> that describe these too. Also, <I>psql
913 -l</I> will list all databases.</P>
915 <P>Also try the file <I>pgsql/src/tutorial/syscat.source</I>. It
916 illustrates many of the <SMALL>SELECT</SMALL>s needed to get
917 information from the database system tables.</P>
919 <H4><A name="4.8">4.8</A>) My queries are slow or don't make use of
920 the indexes. Why?</H4>
921 Indexes are not automatically used by every query. Indexes are only
922 used if the table is larger than a minimum size, and the query
923 selects only a small percentage of the rows in the table. This is
924 because the random disk access caused by an index scan can be
925 slower than a straight read through the table, or sequential scan.
927 <P>To determine if an index should be used, PostgreSQL must have
928 statistics about the table. These statistics are collected using
929 <SMALL>VACUUM ANALYZE</SMALL>, or simply <SMALL>ANALYZE</SMALL>.
930 Using statistics, the optimizer knows how many rows are in the
931 table, and can better determine if indexes should be used.
932 Statistics are also valuable in determining optimal join order and
933 join methods. Statistics collection should be performed
934 periodically as the contents of the table change.</P>
936 <P>Indexes are normally not used for <SMALL>ORDER BY</SMALL> or to
937 perform joins. A sequential scan followed by an explicit sort is
938 usually faster than an index scan of a large table.</P>
939 However, <SMALL>LIMIT</SMALL> combined with <SMALL>ORDER BY</SMALL>
940 often will use an index because only a small portion of the table
941 is returned. In fact, though MAX() and MIN() don't use indexes,
942 it is possible to retrieve such values using an index with ORDER BY
947 ORDER BY col [ DESC ]
951 <P>When using wild-card operators such as <SMALL>LIKE</SMALL> or
952 <I>~</I>, indexes can only be used in certain circumstances:</P>
954 <LI>The beginning of the search string must be anchored to the start
957 <LI><SMALL>LIKE</SMALL> patterns must not start with <I>%</I>.</LI>
958 <LI><I>~</I> (regular expression) patterns must start with
961 <LI>The search string can not start with a character class,
963 <LI>Case-insensitive searches such as <SMALL>ILIKE</SMALL> and
964 <I>~*</I> do not utilise indexes. Instead, use functional
965 indexes, which are described in section <a href="#4.12">4.12</a>.</LI>
966 <LI>The default <I>C</I> locale must be used during
971 <H4><A name="4.9">4.9</A>) How do I see how the query optimizer is
972 evaluating my query?</H4>
974 <P>See the <SMALL>EXPLAIN</SMALL> manual page.</P>
976 <H4><A name="4.10">4.10</A>) What is an R-tree index?</H4>
978 <P>An R-tree index is used for indexing spatial data. A hash index
979 can't handle range searches. A B-tree index only handles range
980 searches in a single dimension. R-trees can handle
981 multi-dimensional data. For example, if an R-tree index can be
982 built on an attribute of type <I>point</I>, the system can more
983 efficiently answer queries such as "select all points within a
984 bounding rectangle."</P>
986 <P>The canonical paper that describes the original R-tree design
989 <P>Guttman, A. "R-trees: A Dynamic Index Structure for Spatial
990 Searching." Proceedings of the 1984 ACM SIGMOD Int'l Conf on Mgmt
993 <P>You can also find this paper in Stonebraker's "Readings in
994 Database Systems".</P>
996 <P>Built-in R-trees can handle polygons and boxes. In theory,
997 R-trees can be extended to handle higher number of dimensions. In
998 practice, extending R-trees requires a bit of work and we don't
999 currently have any documentation on how to do it.</P>
1001 <H4><A name="4.11">4.11</A>) What is the Genetic Query
1004 <P>The <SMALL>GEQO</SMALL> module speeds query optimization when
1005 joining many tables by means of a Genetic Algorithm (GA). It allows
1006 the handling of large join queries through nonexhaustive
1009 <H4><A name="4.12">4.12</A>) How do I perform regular expression
1010 searches and case-insensitive regular expression searches? How do I
1011 use an index for case-insensitive searches?</H4>
1013 <P>The <I>~</I> operator does regular expression matching, and
1014 <I>~*</I> does case-insensitive regular expression matching. The
1015 case-insensitive variant of <SMALL>LIKE</SMALL> is called
1016 <SMALL>ILIKE</SMALL>.</P>
1018 <P>Case-insensitive equality comparisons are normally expressed
1023 WHERE lower(col) = 'abc';
1026 This will not use an standard index. However, if you create a
1027 functional index, it will be used:
1029 CREATE INDEX tabindex ON tab (lower(col));
1032 <H4><A name="4.13">4.13</A>) In a query, how do I detect if a field
1033 is <SMALL>NULL</SMALL>?</H4>
1035 <P>You test the column with <SMALL>IS NULL</SMALL> and <SMALL>IS
1036 NOT NULL</SMALL>.</P>
1038 <H4><A name="4.14">4.14</A>) What is the difference between the
1039 various character types?</H4>
1041 Type Internal Name Notes
1042 --------------------------------------------------
1043 "char" char 1 character
1044 CHAR(n) bpchar blank padded to the specified fixed length
1045 VARCHAR(n) varchar size specifies maximum length, no padding
1046 TEXT text no specific upper limit on length
1047 BYTEA bytea variable-length byte array (null-byte safe)
1050 <P>You will see the internal name when examining system catalogs
1051 and in some error messages.</P>
1053 <P>The last four types above are "varlena" types (i.e., the first
1054 four bytes on disk are the length, followed by the data). Thus the
1055 actual space used is slightly greater than the declared size.
1056 However, these data types are also subject to compression or being
1057 stored out-of-line by <SMALL>TOAST</SMALL>, so the space on disk
1058 might also be less than expected.</P>
1060 <P><SMALL>CHAR(n)</SMALL> is best when storing strings that are
1061 usually the same length. <SMALL>VARCHAR(n)</SMALL> is best when
1062 storing variable-length strings but it limits how long a string can
1063 be. <SMALL>TEXT</SMALL> is for strings of unlimited length, maximum
1064 1 gigabyte. <SMALL>BYTEA</SMALL> is for storing binary data,
1065 particularly values that include <SMALL>NULL</SMALL> bytes. These
1066 types have similar performance characteristics.</P>
1068 <H4><A name="4.15.1">4.15.1</A>) How do I create a
1069 serial/auto-incrementing field?</H4>
1071 <P>PostgreSQL supports a <SMALL>SERIAL</SMALL> data type. It
1072 auto-creates a sequence and index on the column. For example,
1075 CREATE TABLE person (
1081 is automatically translated into this:
1083 CREATE SEQUENCE person_id_seq;
1084 CREATE TABLE person (
1085 id INT4 NOT NULL DEFAULT nextval('person_id_seq'),
1088 CREATE UNIQUE INDEX person_id_key ON person ( id );
1091 See the <I>create_sequence</I> manual page for more information
1092 about sequences. You can also use each row's <I>OID</I> field as a
1093 unique value. However, if you need to dump and reload the database,
1094 you need to use <I>pg_dump</I>'s <I>-o</I> option or <SMALL>COPY
1095 WITH OIDS</SMALL> option to preserve the <SMALL>OID</SMALL>s.
1097 <H4><A name="4.15.2">4.15.2</A>) How do I get the value of a
1098 <SMALL>SERIAL</SMALL> insert?</H4>
1100 <P>One approach is to retrieve the next <SMALL>SERIAL</SMALL> value
1101 from the sequence object with the <I>nextval()</I> function
1102 <I>before</I> inserting and then insert it explicitly. Using the
1103 example table in <A href="#4.15.1">4.15.1</A>, an example in a
1104 pseudo-language would look like this:</P>
1106 new_id = execute("SELECT nextval('person_id_seq')");
1107 execute("INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal')");
1110 You would then also have the new value stored in
1111 <CODE>new_id</CODE> for use in other queries (e.g., as a foreign
1112 key to the <CODE>person</CODE> table). Note that the name of the
1113 automatically created <SMALL>SEQUENCE</SMALL> object will be named
1114 <<I>table</I>>_<<I>serialcolumn</I>>_<I>seq</I>, where
1115 <I>table</I> and <I>serialcolumn</I> are the names of your table
1116 and your <SMALL>SERIAL</SMALL> column, respectively.
1118 <P>Alternatively, you could retrieve the assigned
1119 <SMALL>SERIAL</SMALL> value with the <I>currval()</I> function
1120 <I>after</I> it was inserted by default, e.g.,</P>
1122 execute("INSERT INTO person (name) VALUES ('Blaise Pascal')");
1123 new_id = execute("SELECT currval('person_id_seq')");
1126 Finally, you could use the <A href="#4.16"><SMALL>OID</SMALL></A>
1127 returned from the <SMALL>INSERT</SMALL> statement to look up the
1128 default value, though this is probably the least portable approach.
1129 In Perl, using DBI with Edmund Mergl's DBD::Pg module, the oid
1130 value is made available via <I>$sth->{pg_oid_status}</I> after
1131 <I>$sth->execute()</I>.
1133 <H4><A name="4.15.3">4.15.3</A>) Don't <I>currval()</I> and
1134 <I>nextval()</I> lead to a race condition with other users?</H4>
1136 <P>No. <I>currval()</I> returns the current value assigned by your
1137 backend, not by all users.</P>
1139 <H4><A name="4.15.4">4.15.4</A>) Why aren't my sequence numbers
1140 reused on transaction abort? Why are there gaps in the numbering of
1141 my sequence/SERIAL column?</H4>
1143 <P>To improve concurrency, sequence values are given out to running
1144 transactions as needed and are not locked until the transaction
1145 completes. This causes gaps in numbering from aborted
1148 <H4><A name="4.16">4.16</A>) What is an <SMALL>OID</SMALL>? What is
1149 a <SMALL>TID</SMALL>?</H4>
1151 <P><SMALL>OID</SMALL>s are PostgreSQL's answer to unique row ids.
1152 Every row that is created in PostgreSQL gets a unique
1153 <SMALL>OID</SMALL>. All <SMALL>OID</SMALL>s generated during
1154 <I>initdb</I> are less than 16384 (from
1155 <I>include/access/transam.h</I>). All user-created
1156 <SMALL>OID</SMALL>s are equal to or greater than this. By default,
1157 all these <SMALL>OID</SMALL>s are unique not only within a table or
1158 database, but unique within the entire PostgreSQL installation.</P>
1160 <P>PostgreSQL uses <SMALL>OID</SMALL>s in its internal system
1161 tables to link rows between tables. These <SMALL>OID</SMALL>s can
1162 be used to identify specific user rows and used in joins. It is
1163 recommended you use column type <SMALL>OID</SMALL> to store
1164 <SMALL>OID</SMALL> values. You can create an index on the
1165 <SMALL>OID</SMALL> field for faster access.</P>
1167 <P>O<SMALL>ID</SMALL>s are assigned to all new rows from a central
1168 area that is used by all databases. If you want to change the
1169 <SMALL>OID</SMALL> to something else, or if you want to make a copy
1170 of the table, with the original <SMALL>OID</SMALL>s, there is no
1171 reason you can't do it:</P>
1173 CREATE TABLE new_table(old_oid oid, mycol int);
1174 SELECT old_oid, mycol INTO new FROM old;
1175 COPY new TO '/tmp/pgtable';
1177 COPY new WITH OIDS FROM '/tmp/pgtable';
1180 CREATE TABLE new_table (mycol int);
1181 INSERT INTO new_table (oid, mycol) SELECT oid, mycol FROM old_table;
1183 <P>O<SMALL>ID</SMALL>s are stored as 4-byte integers, and will
1184 overflow at 4 billion. No one has reported this ever happening, and
1185 we plan to have the limit removed before anyone does.</P>
1187 <P>T<SMALL>ID</SMALL>s are used to identify specific physical rows
1188 with block and offset values. T<SMALL>ID</SMALL>s change after rows
1189 are modified or reloaded. They are used by index entries to point
1190 to physical rows.</P>
1192 <H4><A name="4.17">4.17</A>) What is the meaning of some of the
1193 terms used in PostgreSQL?</H4>
1195 <P>Some of the source code and older documentation use terms that
1196 have more common usage. Here are some:</P>
1199 <LI>table, relation, class</LI>
1201 <LI>row, record, tuple</LI>
1203 <LI>column, field, attribute</LI>
1205 <LI>retrieve, select</LI>
1207 <LI>replace, update</LI>
1209 <LI>append, insert</LI>
1211 <LI><SMALL>OID</SMALL>, serial value</LI>
1213 <LI>portal, cursor</LI>
1215 <LI>range variable, table name, table alias</LI>
1218 <P>A list of general database terms can be found at: <A href=
1219 "http://hea-www.harvard.edu/MST/simul/software/docs/pkgs/pgsql/glossary/glossary.html">http://hea-www.harvard.edu/MST/simul/software/docs/pkgs/pgsql/glossary/glossary.html</A></P>
1221 <H4><A name="4.18">4.18</A>) Why do I get the error <I>"ERROR:
1222 Memory exhausted in AllocSetAlloc()"</I>?</H4>
1224 <P>You probably have run out of virtual memory on your system,
1225 or your kernel has a low limit for certain resources. Try this
1226 before starting <I>postmaster</I>:</P>
1232 Depending on your shell, only one of these may succeed, but it will
1233 set your process data segment limit much higher and perhaps allow
1234 the query to complete. This command applies to the current process,
1235 and all subprocesses created after the command is run. If you are
1236 having a problem with the <SMALL>SQL</SMALL> client because the
1237 backend is returning too much data, try it before starting the
1240 <H4><A name="4.19">4.19</A>) How do I tell what PostgreSQL version
1243 <P>From <I>psql</I>, type <CODE>SELECT version();</CODE></P>
1245 <H4><A name="4.20">4.20</A>) Why does my large-object operations
1246 get <I>"invalid large obj descriptor"</I>?</H4>
1248 <P>You need to put <CODE>BEGIN WORK</CODE> and <CODE>COMMIT</CODE>
1249 around any use of a large object handle, that is, surrounding
1250 <CODE>lo_open</CODE> ... <CODE>lo_close.</CODE></P>
1252 <P>Currently PostgreSQL enforces the rule by closing large object
1253 handles at transaction commit. So the first attempt to do anything
1254 with the handle will draw <I>invalid large obj descriptor</I>. So
1255 code that used to work (at least most of the time) will now
1256 generate that error message if you fail to use a transaction.</P>
1258 <P>If you are using a client interface like <SMALL>ODBC</SMALL> you
1259 may need to set <CODE>auto-commit off.</CODE></P>
1261 <H4><A name="4.21">4.21</A>) How do I create a column that will
1262 default to the current time?</H4>
1264 <P>Use <I>CURRENT_TIMESTAMP</I>:</P>
1266 <CODE>CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
1270 <H4><A name="4.22">4.22</A>) Why are my subqueries using
1271 <CODE><SMALL>IN</SMALL></CODE> so slow?</H4>
1273 <P>Currently, we join subqueries to outer queries by sequentially
1274 scanning the result of the subquery for each row of the outer
1275 query. If the subquery returns only a few rows and the outer query
1276 returns many rows, <CODE><SMALL>IN</SMALL></CODE> is fastest. To
1277 speed up other queries, replace <CODE>IN</CODE> with
1278 <CODE>EXISTS</CODE>:</P>
1281 WHERE col IN (SELECT subcol FROM subtab);
1286 WHERE EXISTS (SELECT subcol FROM subtab WHERE subcol = col);
1289 For this to be fast, <CODE>subcol</CODE> should be an indexed column.
1290 We hope to fix this limitation in a future release.
1292 <H4><A name="4.23">4.23</A>) How do I perform an outer join?</H4>
1294 <P>PostgreSQL supports outer joins using the SQL standard syntax.
1295 Here are two examples:</P>
1298 FROM t1 LEFT OUTER JOIN t2 ON (t1.col = t2.col);
1303 FROM t1 LEFT OUTER JOIN t2 USING (col);
1306 <P>These identical queries join t1.col to t2.col, and also return
1307 any unjoined rows in t1 (those with no match in t2). A
1308 <SMALL>RIGHT</SMALL> join would add unjoined rows of t2. A
1309 <SMALL>FULL</SMALL> join would return the matched rows plus all
1310 unjoined rows from t1 and t2. The word <SMALL>OUTER</SMALL> is
1311 optional and is assumed in <SMALL>LEFT</SMALL>,
1312 <SMALL>RIGHT</SMALL>, and <SMALL>FULL</SMALL> joins. Ordinary joins
1313 are called <SMALL>INNER</SMALL> joins.</P>
1315 <P>In previous releases, outer joins can be simulated using
1316 <SMALL>UNION</SMALL> and <SMALL>NOT IN</SMALL>. For example, when
1317 joining <I>tab1</I> and <I>tab2</I>, the following query does an
1318 <I>outer</I> join of the two tables:<BR>
1322 SELECT tab1.col1, tab2.col2
1324 WHERE tab1.col1 = tab2.col1
1326 SELECT tab1.col1, NULL
1328 WHERE tab1.col1 NOT IN (SELECT tab2.col1 FROM tab2)
1332 <H4><A name="4.24">4.24</A>) How do I perform queries using
1333 multiple databases?</H4>
1335 <P>There is no way to query a database other than the current one.
1336 Because PostgreSQL loads database-specific system catalogs, it is
1337 uncertain how a cross-database query should even behave.</P>
1339 <P><I>contrib/dblink</I> allows cross-database queries using
1340 function calls. Of course, a client can make simultaneous
1341 connections to different databases and merge the results on the
1344 <H4><A name="4.25">4.25</A>) How do I return multiple rows or
1345 columns from a function?</H4>
1347 <P>You can return result sets from PL/pgSQL functions using
1348 <I>refcursors</I>. See <A href=
1349 "http://www.PostgreSQL.org/idocs/index.php?plpgsql-cursors.html">
1350 http://www.PostgreSQL.org/idocs/index.php?plpgsql-cursors.html</A>,
1351 section 23.7.3.3.</P>
1353 <H4><A name="4.26">4.26</A>) Why can't I reliably create/drop
1354 temporary tables in PL/PgSQL functions?</H4>
1355 <P>PL/PgSQL caches function contents, and an unfortunate side effect
1356 is that if a PL/PgSQL function accesses a temporary table, and that
1357 table is later dropped and recreated, and the function called
1358 again, the function will fail because the cached function contents
1359 still point to the old temporary table. The solution is to use
1360 <SMALL>EXECUTE</SMALL> for temporary table access in PL/PgSQL. This
1361 will cause the query to be reparsed every time.</P>
1363 <H4><A name="4.27">4.27</A>) What replication options are available?
1365 <P>There are several master/slave replication options available.
1366 These allow only the master to make database changes and the slave
1367 can only do database reads. The bottom of <a
1368 href="http://gborg.PostgreSQL.org/genpage?replication_research">
1369 http://gborg.PostgreSQL.org/genpage?replication_research</a> lists
1370 them. A multi-master replication solution is being worked on at <a
1371 href="http://gborg.PostgreSQL.org/project/pgreplication/projdisplay.php">http://gborg.PostgreSQL.org/project/pgreplication/projdisplay.php</a>.</P>
1373 <H4><A name="4.28">4.28</A>) What encryption options are available?
1376 <LI><I>contrib/pgcrypto</I> contains many encryption functions for
1377 use in <SMALL>SQL</SMALL> queries.</LI>
1378 <LI>The only way to encrypt transmission from the client to the
1379 server is by using <I>hostssl</I> in <I>pg_hba.conf</I>.</LI>
1380 <LI>Database user passwords are automatically encrypted when stored
1381 in version 7.3. In previous versions, you must enable the option
1382 <I>PASSWORD_ENCRYPTION</I> in <I>postgresql.conf</I>.</LI>
1383 <LI>The server can run using an encrypted file system.</LI>
1388 <H2 align="center">Extending PostgreSQL</H2>
1390 <H4><A name="5.1">5.1</A>) I wrote a user-defined function. When I
1391 run it in <I>psql</I>, why does it dump core?</H4>
1393 <P>The problem could be a number of things. Try testing your
1394 user-defined function in a stand-alone test program first.</P>
1396 <H4><A name="5.2">5.2</A>) How can I contribute some nifty new
1397 types and functions to PostgreSQL?</H4>
1399 <P>Send your extensions to the <I>pgsql-hackers</I> mailing list,
1400 and they will eventually end up in the <I>contrib/</I>
1403 <H4><A name="5.3">5.3</A>) How do I write a C function to return a
1406 <P>In versions of PostgreSQL beginning with 7.3, table-returning
1407 functions are fully supported in C, PL/PgSQL, and SQL. See the
1408 Programmer's Guide for more information. An example of a
1409 table-returning function defined in C can be found in
1410 <I>contrib/tablefunc</I>.</P>
1412 <H4><A name="5.4">5.4</A>) I have changed a source file. Why does
1413 the recompile not see the change?</H4>
1415 <P>The <I>Makefiles</I> do not have the proper dependencies for
1416 include files. You have to do a <I>make clean</I> and then another
1417 <I>make</I>. If you are using <SMALL>GCC</SMALL> you can use the
1418 <I>--enable-depend</I> option of <I>configure</I> to have the
1419 compiler compute the dependencies automatically.</P>
projects / postgresql / blob